The present invention relates to a binarization processing method for multivalued image to provide the compatibility of gradation and resolution when multivalued image, expressing the density gradations by multivalues, is reproduced as binary image based on printing dots, and it also relates to a method to form density pattern when binary gradation is reproduced by binary values of white and black (ON and OFF) printing dots from the multivalued image expressing the density gradation by multivalues.
When an image is reproduced, a binary gradation reproduction method is used, by which the density gradation of the image is expressed by changing the ratio of the area of the inked portion in small unit area on the image to be reproduced by ON-OFF control of a plurality of printing dots arranged in longitudinal and lateral directions in matrix form.
According to this method, printing can be achieved much easier compared with the density gradation method, by which the density of ink is changed or the thickness of applied ink is changed to express the gradation of density.
This method is called the area-type density expression method.
The area-type density expression method is roughly divided into dither method and density pattern method. According to the dither method, after the original image is divided into a plurality of pixels in matrix form and the gradation of density for each pixel is expressed by multivalues (e.g. 0, 1, 2, . . . , 255), each printing dot is allocated to one pixel, and the gradation value of the inputted pixel is compared with the threshold corresponding to the position of said printing dot. If it is larger than the threshold, said printing dot is expressed as "black" (ON), and if it is smaller than the threshold, it is expressed as "white" (OFF), thus expressing by two gradations. On the other hand, according to the density pattern method, a group of printing dots of mn in quantity in the matrix form of m.times.n is allocated to one pixel, and it is expressed by one of the gradations of "mn+1" gradations of 0, 1, 2, . . . , mn, which are expressed by the number of printing dots of "dark" (ON) in this group of printing dots.
The dither method provides higher resolution, while the density pattern method has higher gradation expression.
The present invention relates to the latter method.